Lecture 27. Wind Lidar (6) Edge Filter-Based Direct Detection Doppler Lidar

Transcription

1 Lecture 27. Wind Lidar (6) Edge Filter-Based Direct Detection Doppler Lidar q FPI and Fizeau edge-filter DDL q Iodine-absorption-line edge-filter DDL q Edge-filter lidar data retrieval and error analysis q Na-DEMOF DDL with multiple frequencies q I 2 -based Doppler lidar profiling of wind and temp q Summary 1

2 Direct Detection Doppler Lidar q Direct detection Doppler lidars (DDL) convert the Doppler frequency shift to the change of intensity, or intensity ratio, or intensity spatial distribution for wind measurements. q One of the key components for non-resonance DDL is the optical frequency discriminator or frequency analyzer, usually implemented in the lidar receiver if it is not available in the atmosphere. q Current available optical frequency discriminators include (1) Fringe imaging with optical interferometers (Fabry-Perot or Fizeau) (2) Scanning FPI: tune the FPI peak transmission frequency (3) Interferometer edge-filter: the edge of a transmission fringe of an optical interferometer (e.g., Fabry-Perot etalon or Fizeau etalon) (4) Molecular absorption-line edge-filter (e.g., iodine I 2 absorption lines) (5) Atomic absorption-line edge-filter (e.g., Na or K magneto-optic filter) (6) Michelson or Mach-Zehnder interferometer with optical autocovariance q A major difference between resonance DDL and non-resonance DDL lies in where the frequency discriminator is - in the atmosphere or in the receiver chain! Because the Fe Na, and K absorption lines are in the atmosphere, the lidar receiver is allowed to be broadband, rather than the narrowband employed in the non-resonance DDL. 2

3 Edge-Filter DDL q Edge filter is to use either high resolution Fabry-Perot etalons or atomic/molecular vapor cell filters to reject part of the return spectra while passing the other part of the spectra to two different channels. The wind or temperature information is then derived from the ratio of signals from these two channels. N 1 = ξ 1 f m1 (T,P,V R ) N 2 ξ 2 f m2 (T,P,V R ) [Gentry et al., OL, 2000] If V R is known, then T can be derived. If T is known, then V R can be derived. The locking filter channel is to ensure the optimum balance of the Edge 1 and Edge 2 filters (F-P etalons) on the zero Doppler-shifted laser signal. 3

4 Single-Edge vs. Double-Edge q Edge filter has single-edge and double-edge filters. See our textbook Chapter 7 Wind Lidar Direct-Detection Lidar. 4

5 Freq Analyzer: Single-Edge Filter q A Fabry-Perot etalon is usually employed as the edge filter. The etalon is locked to the zero-doppler laser frequency, ν 0, such that the frequency of the transmitted laser is matched to the mid-point of the quasi-linear transmission edge of the etalon. q The intensity ratio of these two channels is a function of the Doppler frequency shift ν s. 5

6 Freq Analyzer: Double-Edge Filter q Two oppositely sloped quasi-linear discriminator edges are used for the two receiver channels in the double-edge design. Usually etalon transmission fringes are used to create the edges. The etalons are locked together (mid-point) to the zero-doppler transmitted laser frequency ν 0. q The intensity ratio of the difference between the two signals to the sum is a sensitive function of the Doppler frequency shift ν s. 6

7 Detectors for FPI Edge-Filter DDL q The information presented to the detector in an edge detection system is the image of the small on-axis solid angle corresponding to the central on-axis fringe of the Fabry-Perot etalon with the necessary spectral FWHM. A suitable detector will be one that has high quantum efficiency, low noise, the capability for photon counting or analog read-out, depending on the intensity of the signal, and which can be time-gated to provide range-resolved information. q The conventional PMT, the APD, and the CCD are among several that have been used successfully, depending on the spectral region of the wind lidar. The PMT is a device that is essentially noise-free when used in photon-counting mode. Due to the negligible read-out and electronic noise, the PMT signal may be post-integrated with complete flexibility, leading to the PMT being widely used as a detector of choice, particularly at 355 and 532 nm. Its drawback is the modest quantum efficiency of the photocathode of the device, normally limited to values of order 40% or less, depending on the spectral region. q 2-D detection: altitude range and time, similar to other lidars, except the fringe-imaging lidars. 7

8 Fringe Imaging vs Edge Filters Multi-Channel PMT or CCD PMT APD CCD 8

9 DDL Based on Fizeau Etalon q DDL based on Fizeau interferometer: linear fringes. Fizeau etalon Fabry-Perot etalon

10 I 2 Absorption Lines Edge-Filter DDL [Liu et al., Appl. Phys. B 64, , 1997] [Friedman et al., Opt. Lett., 22, , 1997] [Liu et al., Appl. Opt., 41, , 2002] [Wang et al., Applied Optics, 49, , 2010] 10

11 Iodine-filter-based Doppler Lidar Two-Axis Scanner Lidar Transmitter Seed laser T & PZT Frequency Control Program 1064 nm 532 nm Photo Diode Nd:YAG Pulsed Laser B.S I 2 filter Photo Diode Beam Expander M PMT I 2 filter Measurement Channel Lidar Receiver B.S P M T IF Reference Channel L FC FC M SCT B.S: Beam splitter; L: Lens M: Mirrors; IF: Interference filter FC: Fiber coupler SCT: Schmidt-Cassegrain Telescope Licel DAQ Data Retrieval Scanner Control Display DAAS Computer Data Acquisition & Analysis System [Wang et al., Applied Optics, 49, , 2010] 11

12 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 2016 Wind Measurements by I2 Doppler Lidar RHI scan (Range Height Indicator) PPI scan (Plan Position Indicator) [Wang et al., Applied Optics, 49, , 2010] 12

13 DDL Data Retrieval and Error Analysis [Wang et al., Applied Optics, 49, , 2010] 13

14 DDL Data Retrieval and Error Analysis Universal and robust error-analysis [Wang et al., Applied Optics, 49, , 2010] 14

15 DDL Data Retrieval and Error Analysis [Wang et al., Applied Optics, 49, , 2010] 15

16 DDL Data Retrieval and Error Analysis [Wang et al., Applied Optics, 49, , 2010] 16

17 Assumptions in Edge-Filter DDL q To derive wind from edge-filter DDL, several quantities have to be taken from models or from independent measurements. q Temperature profile: since the Doppler broadening (depending on temperature) affects the transmitted signal strength, it has to be pre-determined or taken from models for single or doubleedge filters. q Aerosol-scattering ratio also has to be determined independently when in the atmosphere region with aerosols. For example, in the I 2 filter case, tuning the Nd:YAG laser to point A can eliminate aerosol signal thus deriving the aerosol scattering ratio when combined with the reference channel. q Background counts in each channel. q Of course, filter transmission functions have to be known and determined to high precision and accuracy. 17

18 Considerations for DDL q Precision requirement: for δv = 1 m/s velocity precision, the freq measurement precision required for the optical freq analyzer in a DDL is δν = 2(δV)/λ = 5.6 MHz for 355 nm. q Accuracy requirement: accuracy should surpass the precision level. This is usually achieved by monitoring the transmitted laser pulse signal or alternatively measuring the backscatter signal from a stationary or very low velocity target or lock the laser and the filter transmission to each other. q Calibration or accuracy is a main problem for non-resonance DDL, because the burden is on the receiver chain which is variable through time or surrounding conditions, especially in FPI case. q On the other hand, resonance fluorescence Doppler lidars put the discriminator to the atomic absorption lines, which do not change with time. Their receivers can be much simpler. 18

19 Papers on DDL wind measurements with a Cabannes-Mie lidar q She et al., Appl. Opt., 46, , 2007: Comparison between iodine vapor filter and FPI q She et al., Appl. Opt., 46, , 2007: impact of aerosol variations on the iodine filter methods q There are several classic papers on the edge filter techniques providing good insight of the DDL techniques: Ø C. L.Korb, Bruce M. G. and C. Y. Weng, Edge technique: theory and application to the lidar measurement of atmosphere wind, Appl. Opt., 31, , (1992). Ø M. L. Chanin, A. Hauchecorne, A. Garnier and D. Nedeljkovic, Recent lidar developments to monitor stratosphere-troposphere exchange, J. Atom. Sol. Terr. Phys., 56, (1994). Ø C. Flesia and C. L. Korb, Theory of the double-edge molecular technique for Doppler lidar wind measurement, Appl. Opt., 38, (1999). Ø J. McKay, Assessment of a multibeam Fizeau wedge interferometer for Doppler wind lidar, Appl. Opt. 41, (2002).

20 Multiple-Frequency Na/K Double-Edge Magneto-Optic Filter DDL Na Double-Edge Magneto-Optic Filter (Na-DEMOF) q With a 3-freq Na or K Doppler lidar, it is possible to measure wind, temperature, and aerosol simultaneously with a Na-DEMOF or K-DEMOF. [Huang, Chu, Williams, et al., Optics Letters, 34, pp.199, 2009] 20

21 DEMOF with a 3-freq Na Doppler Lidar Calibration curves for ratio technique with Na-DEMOF R W (V LOS,T,R b ) = N R+ N L+ N R+ + N L+ R T (V LOS,T,R b ) = N L N R Ø Temperature and wind are determined simultaneously from two ratios. 21

22 Field Demonstration of Simultaneous Wind and Temperature Measurements (10-45 km) with Na-DEMOF and 3-Frequency Na Lidar 22

23 LIDAR REMOTE SENSING PROF. XINZHAO CHU CU-BOULDER, SPRING 2016 Field Demonstration of Simultaneous Wind and Temperature Measurements (10-45 km) with Na-DEMOF and 3-Frequency Na Lidar [Huang, Chu, et al., Optics Letters, 34, pp. 1552, 2009] 23

24 Doppler Rayleigh Iodine Spectrometer-Based Doppler Rayleigh/Mie/Raman Lidar to Profile Wind and Temperature up to 80 km [Baumgarten, Atmos. Meas. Tech., 3, , 2010] 24

25 Doppler Rayleigh Iodine Spectrometer-Based Doppler Rayleigh/Mie/Raman Lidar to Profile Wind and Temperature up to 80 km Currently the data retrieval is for molecular scattering (Rayleigh) only, but since rotational and vibrational Raman as well as multiple wavelength aerosol scatterings are also detected, in principle aerosol and temperature information can be derived, so can be used to derive Doppler wind more precisely in the aerosol-loaded regions. [Baumgarten, Atmos. Meas. Tech., 3, , 2010] 25

26 Doppler Rayleigh Iodine Spectrometer-Based Doppler Rayleigh/Mie/Raman Lidar to Profile Wind and Temperature up to 80 km [Baumgarten, Atmos. Meas. Tech., 3, , 2010] 26

27 Doppler Rayleigh Iodine Spectrometer-Based Doppler Rayleigh/Mie/Raman Lidar to Profile Wind and Temperature up to 80 km Temperatures are derived from the integration technique combining Rayleigh and VR Raman scatterings. Δt=2 h, ΔV LOS = km, km P L = nm, D telescope = 1.8 m 27 [Baumgarten, Atmos. Meas. Tech., 3, , 2010]

28 Summary q Direct detection Doppler lidar (DDL) uses atomic/molecular absorption lines, the edge filters, or fringe-imaging techniques to discriminate or analyze the frequency or spectrum of the return lidar signals (Doppler shifted and/or broadened). Potentially, DDL can measure both wind and temperature if sufficient spectral information is provided or inquired. q For atmospheric science study, especially for waves coupling from lower to upper atmosphere, DDLs have very high potentials for the future, especially the combination of resonance DDL in MLT region with non-resonance DDL in the troposphere, stratosphere and lower mesosphere, we may be able to profile the wind and temperature from ground all the way up to 120 km. This will be a breakthrough for atmospheric science community. Please read our textbook Chapter 7 for direct-detection Doppler lidar and for coherent-detection Doppler lidar. 28